php - How to deny payment via PayPal IPN? -

I need to create a dynamic 'pay Now' button on my site, and PayPal says that the way to do this is HTML Through the form, with the preset variable for the price, currency and the purchase item. Use PayPal IPN to notify me when the payment is complete. I

However, to change a price, to stop a person from modifying the query parameters of the Pay Now button? Some people have asked me to redirect the button through a PHP file that sends you with the parameter on the paper payment page, but the price can easily be tampered with in the web browser's address bar. My question is, if I get information from PayPal's IPN service is invalid (if the price does not match our records) then how can I refuse payment?

I am quite confused and I do not know what I see but any document is expected, you can help people.

Thank you!